This adds #[derive(Debug)] to all public structs and enums possible.
This also required adding it to some private types that they
transitively depend on. However, the following types depend on types
from external crates that do not implement Debug:
* device::Device
* hub::Hub
* swap_chain::Surface
* swap_chain::SwapChain
To support these types, we would need to use either custom Debug impls
or something like Derivative.
This helps improve the situation in #76.
